So I broke my diff on the dragstrip. Clean break at the notch.

4:11 torsen from an NA in a v6 powered NB.




I have a friend with a TIG. I have aluminum plate.




Can I tig the arm back on, then plate the whole diff arm on both sides to repair?




Is there something else I can do to prevent this from happening again? Solid bushings? A truss assembly to triangulate things from the centersection to the mount bushing?




Give me thoughts, feedback, and options please.

20191028_150204 by Michael Crawford, on Flickr

 

From your picture, it looks like the top of the diff is hitting something.

There are reinforcement plates available on the market, but these are bolt in.
reinforcement plates

I am not sure how easy it is to weld this piece since it is cast. Probably would not hurt too much to try.

 

Besides repairing the arm ( I vote for plates on both sides). Id offer the option of swapping the Diff housing from an RX7 FC (non turbo type). They are supposedly a direct fit and are beefier than a miata diff housing.

 

From the above web link.

"Reinforcing the right mount arm on the 1994-2005 Miata differentials is a common practice in the Miata world because Mazda engineered a failure point into the aluminum housing.

While that may not be a bad thing for a street car, it's also a cause of constant failures on vehicles built for competition. Until now, the only way to reinforce the differential arm was to remove the rear housing, weld on aluminum plates, and reinstall."

 

If your friends a good welder, it’s very easy to weld. All the bolt in plates also double as weld on plates.

 

Mazda Motorsports and other Spec Miata builders sell weld in braces (https://www.miatacage.com/differential-braces). Just the bolt in braces are not enough. Gregg Peters (CarPassionalChannel) has already shown the bolt-in braces don't fix the weakness.

 

Once it's already cracked your best option is to weld it. Our reinforcement plates do work quite well on an undamaged housing. You'll eventually find the limit of any reinforcement if you're launching the car with 400+whp and the PPF is deleted, like Greg Peters

 

If you're talking about the damaged fins and the nub at the top (what exactly is its purpose) that looks ground down, I think you may be right.

 

In the future, you can reduce the potential failure at the notch by shaving down the material around it so it's not a stress concentration. It technically makes the whole arm a tiny bit weaker, but it will likely crack less.

example: black line is as-cast, red line is "stress relieved"
just make sure you make all the material baby butt smooth after you're done.

 

Anyone tried just welding up the notch, without adding plates? I wonder if the cooling stresses would cause more harm than good, or if that would be a fix (on a previously NOT BROKEN housing).

 

The big nub is most likely just a casting mark.
Probably where the inlet/vent for the sand cast.


It's possible that it's a leftover casting from a non-mx5 use for the diff, maybe the RX7 has something bolted there?

 

Housings are cheap. Start with a solid housing and weld on reinforcing plates.

 

A welded arm is going to fail eventually just outside the weld due to fatigue and probable loss in strength due to losing whatever temper the OEM used on the casting. Replacement diff and bolted reinforcement plate gets my vote.

 

I would go to a FC non-turbo housing TBH

 

So i purchased the plates from miatacage after i had my diff break at my last event. I had also purchased a new good 1.8 housing. Got my local welding shop to repair then reinforce the broken diff as a backup, and reinforced my good diff to go into the car.

Here is a question: Does the plate just reinforce the relief's? why not add plates to the other 3 arms to box the diff? Im assuming no one has broken a reinforced diff before or an rx7 housing, anyone know of anyone that has? Also should i upgrade my diff bushing or stock bushing should be fine? Thanks

 

When i plated the replacement carrier, i did all four. O figure overkill is the right amount of kill.